Electromagnetic waves sent at terahertz frequencies, known as terahertz (THz) radiation occur in the region of the electromagnetic spectrum between 100 gigahertz (1×1011 Hz) and several terahertz, and correspond to the wavelength range of from roughly a single digit millimeter (high-frequency edge of the microwave band) to roughly 100 micrometer (long-wavelength edge of far-infrared light).
THz-rays, which are shorter than microwaves and longer than infrared, have potential usage in biomedical and security applications in that THz-rays are safe, and non-ionizing, and can pass through (and enable viewing through) such materials as clothing, paper, cardboard, wood, masonry, plastic, ceramics, as well as penetrate fog and clouds. THz radiation is safe for biological tissues (unlike X-rays), and images can be formed with terahertz radiation having resolution of less than 1 mm. THz radiation has potential spectroscopic uses in that while many materials are transparent to THz, many materials exhibit unique spectral identifiers when exposed to terahertz radiation, including explosives, pharmaceuticals, and illegal narcotic substances. Accordingly, items of interest can be observed through normally visually opaque intervening layers, such as packaging and clothing. For example, airport security scanners may use terahertz radiation.
To date, THz rays have not yet been widely used, partly because of the difficulty in producing them at sufficient intensity.